Making a textured sphere using color coded height map?
Run the color through a "Seperate HSV" node to get the Hue. Then invert the hue and you'll have a more or less accurate result. If you want to make the mountains taller relative to the grasslands I imagine you could do it using RGB curves node. You may also want to get rid of the sea's variation in depth, so that it doesn't look like the sea is uneven. I would recommend doing that by hand or at least without blender. 
Let me add, that you don't really want real displacement. You would need extremely dense geometry and it wouldn't look good either. A planet is so large, that its surface imperfections and bumps can not be noticed from space. At least our planet is like that. A simple bump node would give you the detail you need in the reflections (mainly via fresnel). Hope this helps.